The present invention relates a method of forming a halftone screen, and more particularly to a halftone screen forming method which is capable, when producing a reprodceable halftone dot image by superposing an image signal generated by scanning an original bearing a continuous-tone image on an electrically generated halftone screen signal, of eliminating a periodic pattern in the halftone dot image by forming each of the dots of the halftone screen produced by the above halftone screen signal with an integral number of scanning lines commensurate with the screen angle of the halftone screen.
Recently, image scanning reading and recording apparatus are widely used in the field of printing and graphic art for electrically processing image information of originals to produce original film plates with a view to simplifying the entire process and improving the quality of printed images.
The image scanning reading and recording apparatus have been basically constructed of an input unit, a control unit, and an output unit. In the input unit, an original delivered to an image reader is scanned by a laser beam or the like, and image information of the original is converted to an electric signal representative of different intensities of reflected laser light. The photoelectrically converted image information is then processed in the control unit for tone correction, outline emphasis, or the like according to platemaking conditions. The image information which is processed by the control unit is thereafter fed to the output unit in which it is converted again to a light signal such as a laser beam. The light signal is applied to a recording medium such as a photosensitive material to record the image thereon. The image on the recording medium is then developed by a developing device, after which the recording medium is used as an original film plate.
Where an original to be printed or reproduced bears a continuous-tone image such as a photograph or a painting, it is necessary to break up the original image into halftone dots in order to reproduce tone gradations clearly. More specifically, a continuous-tone image is converted to a halftone dot image which is composed of closely spaced dots of different sizes according to gradations of density of the image. One method of breaking up a continuous-tone image into halftone dots is to apply a light signal commensurate with the continuous-tone image to a film through a contact screen having a vignetted dot pattern. The aforesaid image scanning reading and recording apparatus employ a process for electrically forming a halftone screen corresponding to such a contact screen.
One preferred example of a conventional method of forming a halftone screen will briefly be described below based on the invention disclosed in Japanese Patent Publication No. 52-49361 claiming Convention Priority based on West German Patent Application No. 1901101.9.
FIG. 1 of the accompanying drawings shows a basic periodic portion 2 of a halftone screen which is electrically formed. The halftone screen is composed of repetitions of one pattern, and the minimum unit of the halftone screen is the basic periodic portion 2. The basic periodic portion 2 is constructed of eight scanning lines S1 through S8 arranged side by side in the direction Y. Each of the scanning lines S1 through S8 forms an element of the basic periodic portion 2 with an inherent voltage signal that varies in the recording direction X. The voltages of the scanning lines S1, S2, S4, S5 passing through points A through D in a dot 4 are high, and the scanning line S3 passing through a point E is low. The voltages of the sanning lines S1 through S5 are selected to become gradually lower from the points A through D to the point E. The voltage signals or halftone screen signals of the scanning lines S1 through S8 may be produced by superposing a plurality of alternating voltage signals of triangular waveform which have different periods, and slightly shifting the phases of those voltage signals.
When converting a multicolor image or the like to dots, it is necessary to generate a plurality of halftone screens and superpose halftone dot images produced respectively by those halftone screens. In order to prevent a moire pattern from being produced when the halftone screens are superposed, the halftone screens are formed while they are rotated a prescribed angle .theta. with respect to the recording direction X.
The basic periodic portion 2 is periodically produced frequently enough to cover a scanned region of the original, for thereby producing a halftone screen. The halftone screen signals constituting the halftone screen are superposed on an image signal that has optically been read from the original by the input unit of the image scanning reading and recording apparatus, so that a halftone-dot image is formed on a film plate.
The halftone screen signals can sufficiently accurately represent the gradations of the halftone screen in the case where the width of each of the scanning lines S1 through S8 of the basic periodic portion 2 is sufficiently smaller than the width of the basic periodic portion 2 in the direction Y and hence the voltages of the halftone screen signals continuously vary in the direction Y. However, if the width of each of the scanning lines S1 through S8 is not negligible with respect to the width of the basic periodic portion 2 in the direction Y, the voltages of the halftone screen signals vary in a step-like discrete manner between the adjacent scanning lines S1 through S8. In such a case, the voltages at the points A through D, which are to be of maximum values, may gradually vary in the scanning lines S1, S2, S4, S5, and such a voltage variation may cause a periodic pattern to be produced in the basic periodic portion 2. Should such a periodic pattern be large enough to be visually conspicuous, an unsightly pattern, other than the gradations of the original, would be introduced into the halftone dot image as with a moire pattern.